Ocean heat store makes climate change inevitable

No matter how well the world controls emissions of greenhouse gases, global climate change is inevitable, warn two new studies which take into account the oceans' slow response to warming.

Even if greenhouse gases never rise beyond their present level, temperatures and sea levels will continue rising for another century or more because of a time lag in the oceans' response to atmospheric temperatures, say researchers.

This time lag means policymakers cannot afford to wait to tackle climate change until its consequences become painful, because by then they will already be committed to further change, they urge. "The feeling is that if things are getting bad, you hit the stop button. But even if you do, the climate continues to change," says Gerald Meehl, a climatologist at the US National Center for Atmospheric Research (NCAR) in Boulder, Colorado.

Meehl and his colleagues used two sophisticated computer models of global climate to predict what would happen under various scenarios for greenhouse gas emission controls, taking into account the oceanic time lag. Their most optimistic scenario - in which atmospheric concentrations of greenhouse gases are capped at year 2000 levels - would require severe cuts in CO2 emissions, far beyond those set in the Kyoto protocol.

But even this optimistic scenario predicts that global temperatures would continue to rise by between 0.4°C and 0.6°C over the next century. That increase is comparable to the increase in global temperature during the 20th century of about 0.6°C. A second, independent study using a simpler climate model by Tom Wigley, another climatologist at NCAR, paints the same bleak picture.

Thermal inertia

The time lag occurs because rising air temperatures take time to make themselves felt throughout the immense thermal mass of the oceans. This "thermal inertia" means that Earth has not yet felt the full effect of today's level of greenhouse gases, explains Meehl.

And because water expands as it warms, this time lag in temperature will continue to push sea level higher. Meehl's models predict that thermal expansion alone would make sea levels rise by about 11 centimetres over the next century, even if greenhouse gases were held at 2000 levels.

The real rise would almost certainly be more, he says, because his models do not include the effect of melting of glaciers and icecaps, which will be more rapid in a warmer world.

In fact, the climatic time lag may be pushing Earth closer to a catastrophic glacial melting, scientists warn. James Hansen, a climatologist at NASA's Goddard Institute for Space Studies in New York, US, has estimated that global temperature may need to rise just 1°C more to tip the balance toward abrupt melting. "That's why the warming already 'in the pipeline' is important," he says. "It takes us closer to the slippery slope."

"We have got climate change in store no matter what," agrees Andrew Weaver, a climatologist at the University of Victoria, Canada. "We have to be prepared to assist those who cannot adapt."

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